This proposal investigates the role of an RNA-binding oncoprotein in mammary tumorigenesis. This protein is located in the cytoplasm and binds to insulin-like growth factor-ll, p-actin, c-myc, and other mRNAs. We call it the c-myc Coding Region instability Determinant-binding Protein or CRD-BP. because it binds to an instability determinant in the c-myc mRNA coding region. The CRD-BP affects RNA translation, stability, and localization, depending on the RNA to which it binds. It is expressed abundantly in fetaltissues but is repressed shortly after birth. Therefore, it probably has a special role in fetal life. Three findings also link the CRD-BP to breast cancer: (a) The CRD-BP is undetectable in normal breast tissue but is present in -60% of human breast tumors, (b) The CRD-BP gene is amplified in -30% of human breast tumors, (c) The CRD-BP induces mammary adenocarcinomas in female transgenic mice. Three aims use transgenic mice to investigate the pathways of mammary tumor induction by this RNA-binding oncoprotein. Aim I. Determine if CRD-BP expression is required continuously to maintain an existing mammary tumor. Mammary tumors in transgenic mice continue to express the CRD-BP and thus might be CRD-BP- dependent for their survival. To investigate this idea, Aim I exploits a mouse model in which mammary- specific CRD-BP expression is induced by doxycycline (dox), a tetracycline analog. Mammary tumors are expected to arise in dox-treated mice. Aim I then asks: Do these tumors stop growing or regress when dox is withdrawn? Our working hypothesis is that tumors will regress when CRD-BP expression is repressed. Aim II. Use microarray analysis to compare gene expression profiles in nori-neoplastic mammary tissues and in purified mammary epithelial cells of mice that do or do not express the CRD-BP. Some CRD- BP-regulated RNAs could play central roles in the induction and maintenance of mammary tumors. Aim III. Determine how genetic and environmental factors contribute to mammary tumorigenesis in CRD-BP-expressing mice. The goal is to assess how the tumor suppressor Ape and the chemical carcinogen N-ethyl-N-nitrosurea (ENU) affect CRD-BP-mediated tumor formation. We predict Apedeficiency and ENU treatment will synergize with the CRD-BP to increase tumor incidence and malignancy. Many of the well-studied oncoproteins are transcription factors or protein kinases. The CRD-BP is an unusual oncoprotein, because it resides in the cytoplasm and binds to RNA. By investigating the oncogenic properties of the CRD-BP, the RNAs it affects in vivo, and its interactions with tumor-inducing genetic and environmental factors, we hope to uncover novel tumorigenesis pathways.